1. Field of the Invention
This invention relates to a cathode structure for an electron tube. In particular the invention is directed to a cathode body fixing structure for a high current density type electron tube. The fixing structure prevents deformation of the cathode body for the electron tube caused by heat generated by a heater, or burnout of the heater caused by electric field concentration.
2. Description of the Prior Art
Recently, electron tubes have been made to have a large size and a higher brightness than previously. To this end, there have been actively made the studies cathodes for high current density type electron tubes. Generally, as illustrated in FIGS. 1 and 2, a cathode body comprises a cathode cap 16 having an electron emission material 17 which includes high temperature, heat resisting metal powder such as tungsten(W) and an oxide such as BaO, CaO, or Al.sub.2 O.sub.3, and a hollow sleeve 11 made of high temperature, heat resisting metal powder such as molybdenum (Mo). There is also a heater 15 mounted within the sleeve 11. The cathodes for the high current density electron tubes require cathode current density of about 10 A/cm.sup.2, and there it is required to obtain such current density at an operating temperature of about 1000.degree. C.b (brightness temperature). In particular, since a very higher activating temperature, of about 1200.degree. C.b is required, the cathodes have exhibited certain disadvantages in that the higher activation temperature results in increased power consumption of the heater, structural deformation of cathode body and burnout of the heater.
With a view to overcoming this problem, there has been proposed a cathode body fixing structure as shown in FIGS. 1 and 2, which comprises a plurality of spaced-apart metallic fixing pieces or metallic ribbons 13 interconnecting the outer peripheral surface of the lower end of the sleeve 11 and the top of a cathode holder 12. With this construction, when a rated voltage is applied to the heater 15 of the cathode body, the temperature of the heat generation section of the heater is raised to a given temperature, and thus the heat is transferred to the electron emission material 17 in the cathode cap 16, so that thermions of predetermined density are emitted from the surface of the electron emissions material. At this time, since the heater 15 rapidly expands because of heat toward the electron emission material 17 located at the top of the cathode body, the sleeve 11 of the cathode body also expands toward the material 17 because of heat transferred to it. As a result, the metallic fixing pieces 13 connecting the cathode body to the cathode holder 12 to support the cathode body undergo deformation because of the heat transferred to them. The reason for this is that the speed of the thermal expansion of the heater 15 is much greater than that of the sleeve 11 and the metallic fixing pieces 13.
In the prior art cathode body fixing structure, since a plurality of the metallic fixing pieces 13 between the sleeve 11 and the cathode holder 12 are disposed in laterally spaced relation to each other, the heat transferred from the heater 15 to the sleeve is not readily conducted to the cathode holder, but is effectively conducted to the electron emission material 17 generating thermions. Therefore, the efficiency of the heater is increased, and this somewhat contributes to reduction in power consumption of the heater.
This prior cathode body fixing structure however has a drawback in that since the metallic fixing pieces 13 interconnecting the sleeve and the cathode holder are inherently weak, as the temperature of the cathode body is raised to a high temperature depending upon heat generated by the heater, the fixing pieces coupled to the sleeve are apt to be easily deformed because of thermal expansion of the sleeve from the heat transferred by the heater.
Further, since the joined portions of the lower ends of the fixing pieces and the lower end of the sleeve are discrete, to form portruded portions on the outer periphery of the sleeve, electric fields may be concentrated at the protruded portions to cause electric discharge between the sleeve and the heater. This structure results in a burnout of the heater.